FIG. 1 depicts schematically parts of a power train for a motor vehicle 1, such as a passenger car or a heavy vehicle, e.g. a truck or bus. The power train comprises an engine 10 mechanically connected by a shaft to a first end of a gearbox 20 via a clutch device 40. The gearbox 20 is also mechanically connected, at its other end, by a propeller shaft 50 to a differential gear 30 associated with a rear axle. The rear axle comprises respective left and right drive shafts 60 which drive the vehicle's powered wheels (not depicted in the diagram).
With this well-known arrangement, the mechanical work of the engine 10 is transmitted via various transmission devices, e.g. the clutch device 40, the gearbox 20, the propeller shaft 50, the differential gear 30 and the drive shafts 60, to powered wheels in order to move the vehicle 1. An important device in the power train is the gearbox 20, which has a number of forward gears for moving the vehicle 1 forwards, and usually also one or more reverse gears. The number of forward gears varies but modern kinds of trucks are usually provided with twelve forward gears.
The gearbox 20 may be of manual or automatic type (automatic gearbox), but also of the automatic manual gearbox type (automatic manual transmission, AMT).
Automatic gearboxes and automatic manual gearboxes are automated gearbox systems usually controlled by a control unit 110 (sometimes also called electronic control unit, ECU) which is adapted to controlling the gearbox 20, e.g. during gear changing, as when choosing appropriate gears at a certain vehicle speed with a certain running resistance. The ECU may measure engine speed and the state of the gearbox 20 and control the gearbox by means of solenoid valves connected to compressed air devices. Information about the engine 10, e.g. its speed and torque, is also sent from the engine 10 to the ECU, e.g. via a CAN (controller area network) bus.
The control unit 110 further comprises devices for receiving information, e.g. in the form of input signals, from the gearbox 20 via a connection 80 and/or from, for example, one or more input units 120 via a connection 90. The control unit further comprises devices for delivering information, e.g. in the form of control signals, to the gearbox 20 via a connection 70 and/or to one or more output units 130 via a connection 100. The control unit may be situated close to the driving cab or close to the gearbox or substantially anywhere in the vehicle 1. Input units 120 and/or output units 130 are with advantage so situated that they are reachable and/or viewable by a driver of the vehicle.
In conventional gear change systems, the control unit 110 uses tabulated engine speed limits when choosing appropriate gears. These engine speed limits are also called shift points and they represent the engine speed at which a downshift or upshift should be effected in the gearbox 20. This means that the vehicle 1 changes gear when the speed of its engine 10 passes a speed represented by a shift point. The shift points are therefore to be construed as providing information not only about when a downshift or upshift should take place but also about the number of gear steps to be effected at each downshift or upshift. It is usual for each shift point to comprise one to three gear steps, but more steps are possible, e.g. one to six.
In certain systems for automated gearboxes, e.g. a system such as referred to in U.S. Pat. No. 5,908,369, a driver can manually choose appropriate gears after having switched the automated gearbox over to manual mode. In that system, the gear lever and the system are so configured that the driver has to switch to manual mode in order to be able to change gear manually, which he/she may find burdensome. In manual mode, the system uses a number of timers to keep track of whether manual gear changing may be effected or not.
There are also certain systems for automated gearboxes in which a driver in automatic mode may be enabled to change gear up or down manually if he/she is not satisfied with the gear choice made in automatic mode. Such manual gear changing is then effected if an input unit 120, here in the form of a mode selector, e.g. a paddle, is activated by a driver of the vehicle, whereupon a manual gear change is imposed.
Such imposed manual gear changing may cause problems in certain situations. If for example the driver imposes a manual gear change near in time to an automatic gear change effected by the automated gearbox, the manual gear change initiated and imposed by him/her risks being added to the automatic gear change. If this happens, the vehicle will change gear up or down more than the driver intended and also more than the automatic shift was intended to do. In other words, the result is an unwanted double gear change which will greatly affect the engine's speed and may cause it to stall.